Volatile insect repellents disappear relatively rapidly when applied to the skin. They are also quickly washed away by water, whether fresh or salt containing.
The most prominent insect repellent in use today is N,N'-diethyl toluamide, commonly known as DEET. Other volatile insect repellents are known including ethyl hexanediol; 2-(octylthio)ethanol; dimethyl phthalate; di-n-propyl-2,5-pyridine dicarboxylate; 1,5a,6,9,9a, 9b-hexahydro-4a(4b)-dibenzofuran carboxaldehyde; citronellal; citronellol; geraniol; nerol; and linalool. The formulation of such insect repellents is particularly problematic due to the greasy feel of many of the repellents and especially the effect of DEET in staining clothing, crazing plastics and washing away in humid or rainy weather or when the person using the repellent is participating in water sports such as swimming or fishing. In addition, the lack of retention of insect repellents due to the action of water is also affected by the individual wearer's sweating.
The evaporation of the insect repellent is directly related to the ambient temperature and wind velocity. Approximately 50% of a topically applied dose is absorbed in six hours with peak plasma levels being reached in 1 hour (Lurie et al, Pharmacokinetics of insect repellent N,N-diethyl toluamide. Med. Parazitol., 47, 72, 1979).
Mehr et al, (Laboratory Evaluation of Controlled-Release Insect Repellent Formulations, J. Am. Mosq. Control Assoc., 1,143, 1985) evaluated a number of controlled release formulations of microencapsulated DEET and hydrophilic vinyl polymers such as polyvinylpyrrolidone. The polyvinylpyrrolidone formula was no better than unformulated DEET in repelling mosquitoes.
Reifenrath et al, (Evaporation and Skin penetration characteristics of mosquito repellent formulations. J. Am. Mosq. Control Assoc., 5, 45 1989) tested silicone polymers, acrylate polymers, fatty acids and mixtures of repellents and evaluated evaporation and skin penetration. No differences in evaporation and skin penetration was found between formulations containing the polymers and unformulated DEET or with a mixture of dimethyl phthalate and DEET.
Reifenrath et al, (In vitro skin evaporation and penetration characteristics of mosquito repellents. J. Pharm. Sci. 71, 1014, 1982) showed that the duration of repellent efficacy on man correlated with the time that vapor levels at the surface of the skin exceeded the minimum effective evaporation rates in vitro.
U.S. Pat No. 4,474,081 discloses the use of maleic anhydride/alpha olefin polymers and terpolymers to provide slow release of contact insect repellents when applied to the surface of the skin.
U.S. Pat No. and U.S. Pat. No. 4,774,082 discloses the use of maleic anhydride/alpha olefin polymers and terpolymers to provide slow release of volatile insect repellents when applied to the surface of the skin.
Chemical Abstracts 110, 207847s (1989), discloses mosquito repellent compositions which have an active agent and an oil-soluble, water insoluble acrylate polymer comprising acrylic acid, stearyl methacrylate and isooctyl acrylate.
Ideally, an insect repellent formulation for mammalian use should be non-staining, non-greasy, long lasting, and resistant to washing off from rain, humidity, sweat, fresh waters or ocean waters and reduce penetration of the skin.